Device for adjusting circuits before encapsulation

ABSTRACT

A device for adjusting an integrated circuit before encapsulation includes a first MOS transistor having a gate and a source connected together, and a body connected to a voltage reference. A first resistor is connected in parallel with the first MOS transistor. A second MOS transistor is connected in series with the first MOS transistor. The second MOS transistor has a gate and a source connected together, and a body connected to the voltage reference. A second resistor is connected in parallel with the second MOS transistor. A first terminal is connected to the source of the first MOS transistor, and a second terminal is connected to the source of the second MOS transistor. The first terminal is acessible externally after the integrated circuit has been encapsulated.

[0001] The present invention concerns the field of analog and digital integrated circuits. These circuits must utilize the smallest possible silicon area in order to reduce costs while still maintaining high precision.

[0002] A silicon wafer that has undergone various steps of etching and/or deposition of conductive, semiconductive or insulating layers is put through a sorting step designed to remove defective circuits. The sorting step is followed by a packaging or encapsulation step.

[0003] During the sorting step, each circuit on a wafer is tested to check its conformance to specifications. A circuit can be considered satisfactory, a reject, or, alternatively, a candidate for adjustment. Adjustment is performed by imposing given electrical voltages and/or currents on bumps of the integrated circuit, some of which bumps may no longer be accessible once the circuit is encapsulated.

[0004] The present invention proposes a device that enables an integrated circuit to be adjusted before encapsulation, during the sorting step.

[0005] The present invention proposes a device forming part of an integrated circuit and disposed between an external contact bump accessible even after encapsulation and the rest of the circuit, that is, its useful portion designed to perform a specific function.

[0006] According to one aspect of the invention, the integrated electronic circuit comprises a plurality of MOS transistors. The circuit comprises at least a first and a second MOS transistor arranged in series, each transistor comprising a grid and a short-circuit source, and a base connected to the ground of the integrated circuit.

[0007] The circuit advantageously comprises a first resistance mounted in parallel with the first transistor and a second resistance mounted in parallel with the second transistor.

[0008] In one embodiment of the invention, the circuit comprises a third transistor mounted in series with the first and second transistors and comprising a grid and a short-circuit source, and a base junction connected to the ground of the integrated circuit.

[0009] The circuit can comprise a third resistance mounted in parallel with the third transistor. This is the global ground of the circuit, which is necessary for its satisfactory operation.

[0010] In one embodiment of the invention, the circuit comprises a connection bump that is connected to the source of the first transistor and is accessible after the circuit is encapsulated. Alternatively, a resistance can be mounted between the bump and the source of the first transistor.

[0011] In one embodiment of the invention, the circuit comprises a connection bump that is connected to the source of the second transistor and is not accessible after the circuit is encapsulated, and a connection bump that is connected to the drain of the second transistor and is not accessible after the circuit is encapsulated.

[0012] In one embodiment of the invention, the circuit further comprises a connection bump that is connected to the drain of the third transistor and is not accessible after the circuit is encapsulated. More generally, the connection bump connected to the drain of the nth transistor can be connected to the rest of the circuit. The term “series-arranged MOS transistor” is understood to mean transistors in which the source of the n+1th [sic] transistor is connected to the drain of the nth transistor.

[0013] MOS transistors may be isolated or nonisolated transistors. The base connector is preferably adjacent the drain.

[0014] The invention also proposes a method of adjusting electrical resistance in an integrated electronic circuit comprising a plurality of series-mounted MOS transistors, each provided with a parallel-mounted resistance. The bases of the MOS transistors are connected to one another. A first voltage is applied to a MOS transistor at its base, its grid and its source and a second voltage is applied to its drain in order to break down said MOS transistor.

[0015] The bases of the MOS transistors are preferably connected to the global ground of the circuit, and the bases of the transistors are short-circuited to the grid and to the source of the MOS transistor that is to be broken down.

[0016] The first voltage is preferably constant and the second voltage is a monotonic ramp. The first voltage can be zero and the second voltage can be increasing.

[0017] The breakdown of the MOS transistor can be effected by avalanche of the drain/substrate junction, irreversible breakdown of the drain/substrate junction and a short-circuit between the drain and the source. The difference between the first and second voltages is about 16 V. The breakdown current can be less than 100 mA.

[0018] The invention applies to both n-MOS and p-MOS transistors.

[0019] The use of “snapback” MOS transistors makes it possible to achieve a short-circuit and thus a resistance inside an integrated circuit by acting on the pins of the integrated circuit that can be accessed prior to encapsulation. A component realized in this manner takes up little space on a silicon wafer and is therefore inexpensive. The fact that the grid and the source of the MOS transistor are short-circuited ensures permanent blocking of said MOS transistor and keeps it from affecting the operation of the rest of the electronic circuit. After breakdown, the MOS transistor can be considered the equivalent of an open circuit.

[0020] The invention makes use of a natural characteristic of MOS transistors, that of having parasitic components, particularly a bipolar transistor. In some configurations, such parasitic components are harmful. During electrostatic discharge, circuits can be seriously damaged by turn-on of the parasitic transistor.

[0021] Conversely, the invention utilizes the parasitic bipolar transistor of the MOS transistor to make it a short circuit and obtain a resistance of a predetermined value between the drain and the source of the MOS transistor, i.e., between the collector and the emitter of the parasitic bipolar transistor. This component can be considered an “antifuse.” A fuse is a closed circuit in the normal state and an open circuit after breakdown. Here, the MOS transistor is an open circuit before breakdown and a closed circuit after breakdown, with a low residual resistance value.

[0022] The present invention will be better understood from a study of the detailed description of a few embodiments, taken strictly as non-limiting examples and illustrated by the appended drawings, wherein:

[0023]FIG. 1 is a characteristic operating curve of a MOS transistor;

[0024]FIG. 2 is a cross-sectional view of a MOS transistor;

[0025]FIG. 3 is a diagram of the device according to the invention; and

[0026]FIG. 4 is a cross-sectional view of a variant MOS transistor.

[0027] As can be seen in FIG. 1, where the drain voltage is plotted on the abscissa and the drain current on the ordinate, an n-MOS transistor has four operating regions. Region 1 is that of the conventional linear operation of a MOS transistor. Region 2 is that of saturation-mode operation, in which current varies only very slightly with voltage. Region 3 is known as the “avalanche” region, with a weakening of the drain/substrate junction caused by avalanche of said junction. Finally, Region 4 is that of turn-on of the parasitic bipolar transistor, with the curve showing a first breakdown, referenced 5, which is reversible, and a second breakdown, referenced 6, which is destructive and therefore irreversible.

[0028] Beyond the second breakdown 6, it will be noted that the current varies extremely rapidly with voltage, the slope of the curve being almost vertical. Since the breakdown process, also known as “second breakdown,” is irreversible, it is possible to move along the curve beginning at the second breakdown 6 by moving up, which translates into a decrease in the resistance offered by the MOS transistor broken down in this way, insofar as the current can be seen to increase against a substantially constant drain voltage.

[0029]FIG. 2 shows the structure of the various constituents. The MOS transistor comprises a drain 8, a source 9 and a grid 10 formed on a base 11, also known as the “bulk.” A parasitic bipolar transistor 12 forms in base 11: its collector is formed by drain 8 and its emitter by source 9, and its base can be modeled as connected to ground by a substrate resistance 13 and by a current source 14 connected to drain 8.

[0030] In the arrangement according to the invention, drain 8 is connected to a first supply voltage, while source 9, grid 10 and base 11 are short-circuited and are connected to a second supply voltage. When the MOS transistor reaches saturation, a high voltage on the drain triggers avalanche of the drain/base junction by generating electron-hole pairs, thus creating a base current. The voltage at the terminals of the base resistance increases, thereby polarizing the source/base junction. The parasitic bipolar transistor thus undergoes flashover and the phenomenon of breakdown occurs.

[0031] At high currents the component goes into the irreversible second breakdown state, represented by destruction of the polysilicon crystal lattice of the channel formed between the drain and the source. After avalanche of the collector/base junction of the parasitic bipolar transistor, the emitter connected to ground serves to forward-bias the base/emitter- junction, which causes the snapback effect. To trigger the avalanche phenomenon, a sufficient voltage must be imposed on the drain to reverse-bias the drain/base junction. This voltage depends on the doping characteristics and is proportional to the square of the electrical field.

[0032] The current generator 14 shown in FIG. 2 between the collector and the base of the parasitic bipolar transistor simulates the leakage currents of the drain/base junction in an initial phase. Thereafter, it serves to simulate avalanche of the junction and polarization of the parasitic bipolar npn-type transistor.

[0033] By way of example, tests were performed using HF4 CMOS technology with an n-MOS transistor having the following channel dimensions: width (W)=1 μm, length (L)=0.7 μm. The source was grounded, and a voltage ramp ranging from 8 to 18 V with current limitation was applied to the drain. With a current of 2 mA, a post-breakdown resistance of 300 Ω was created. With a current of 10 mA, a post-breakdown resistance of 60 Ω was obtained, and with a current of 100 mA, a post-breakdown resistance of 11 Ω. It will be noted that with a drain voltage of less than 11 V, the drain/base junction is not in avalanche, and therefore no current passes through the drain/source channel. Beyond this voltage the phenomenon sets in, with the creation of a conductive path allowing the passage of current. Once the breakdown voltage is reached, all the available current flows into the channel and a resistance is created.

[0034] It is particularly advantageous to use transistors whose channel is as short as possible, since the shorter the channel, the lower the breakdown voltage, due to the increase in the drain current and the increase in the number of electron-hole pairs generated, the channel width being constant. A decrease in channel width brings about a decrease in the voltage and the current of the second breakdown 6 illustrated in FIG. 1. Even if the width of the channel has no effect on the voltage of the first breakdown 5, a reduced width will increase the heating effect of the second breakdown 6, since the lines of force will be more unidirectional, implying a decrease in the torque of the second breakdown. It is therefore particularly advantageous to use small-sized MOS transistors.

[0035] When a MOS transistor is used in snapback mode, the substrate is connected to the lowest potential of the circuit in order to reverse-bias all the parasitic diodes existing between drain 8 and source 9, on the one hand, and base 11 on the other. Source 9 and base 11 are short-circuited. Grid 10 is also short-circuited to source 9 and to the base in order to deactivate the transistor.

[0036] Resistances arranged in parallel can be adjusted with this type of snapback MOS transistor; see, on this subject, the document FR A 2,795,557.

[0037]FIG. 3 shows an embodiment of the invention comprising three series-arranged resistances to be adjusted, referenced 15, 16 and 17, resistance 15 being connected to a ground bump 18 that will be connected to one of the external pins of the circuit at the time of encapsulation, and resistance 17 being connected to the rest of the circuit (not shown). The device further comprises three MOS transistors 19, 20, 21, each provided with a grid, respectively 22, 23 and 24, a drain, respectively 25, 26 and 27, a source, respectively 28, 29 and 30, and a base, respectively 31, 32 and 33.

[0038] Transistor 19 is mounted in parallel with resistance 15, transistor 20 in parallel with resistance 16 and transistor 21 in parallel with resistance 17. The grid and the source of each transistor 19, 20 and 21 are short-circuited. Bases 31 to 33 of transistors 19 to 21 are all connected to bump 18. Grid 22 and source 28 of transistor 19 are connected to bump 18. Drain 25 of transistor 19 and grid 23 and source 29 of transistor 20 are connected to the common point between resistances 15 and 16 and to an adjustment bump 34 that can be no longer accessible after the circuit is encapsulated. Drain 26 of transistor 20 and grid 24 and source 30 of transistor 21 are connected to the common point between resistances 16 and 17 and to an adjustment bump 35 that can be no longer accessible after the circuit is encapsulated. Drain 27 of transistor 21 is connected to the other terminal of resistance 17, to the rest of the circuit (not shown) and to an adjustment bump 36 that can be no longer accessible after the circuit is encapsulated.

[0039] To break down transistor 21, bumps 18 and 35 are together connected to ground and a positive voltage ramp is applied to bump 36. If bump 18 were left unconnected, the snapback phenomenon would not occur due to the impossibility of avalanching the drain/base junction and thus of forward-biasing the base/transmitter junction.

[0040] If transistor 20 is to be broken down, bumps 18 and 34 are connected together and a positive voltage ramp is applied to bump 35.

[0041] To break down transistor 19, a positive voltage is applied to bump 34 and bump 18 is connected to ground.

[0042] A system of low-value series-connected resistances can thus be adjusted by means of MOS transistors in a reproducible and reliable manner. A nonisolated MOS transistor takes up much less area than an isolated MOS transistor, for example an area of 7 μm out of 14 μm instead of 40 μm out of 40 μm, thus dividing the occupied silicon area by about 16.

[0043] Breakdown of the isolated snapback MOS transistors is difficult to achieve due to the presence of a second, parasitic bipolar transistor in which the collector is formed by the drain of the MOS transistor, the emitter by the source and the base by the bulk. This second, parasitic transistor being [sic] capable of stealing most of the current sent by a breakdown bump to the drain of the MOS transistor. As a result, the post-breakdown resistance can vary between 100 Ω and 1 kΩ, compared to the resistance of 10 Ω that is obtained in a reproducible manner when the nonisolated MOS structure is broken down.

[0044] The term “isolated MOS transistor” is understood here to mean a MOS transistor whose substrate and base are separated by a dielectric layer. Vertical and especially lateral isolation stresses encourage the use of large silicon areas in order to increase the dimensions of the base and decrease the gain of the second, parasitic bipolar transistor.

[0045] Without excluding isolated MOS transistors, it is preferred to use nonisolated MOS transistors that prevent the flow of leakage currents from the base to the substrate. The base and the substrate are at the same potential. Further, it is particularly advantageous to arrange the base connector as close as possible to the drain for reasons of current line distribution during breakdown. The base layer, a p-type layer in an n-MOS transistor, may or may not be ring-shaped. In both cases, the base connector, i.e., the lead-out to the connection levels, is arranged closer to the drain than to the source.

[0046] The invention thus enables series-arranged resistances to be adjusted in a precise and reproducible manner with the aid of economical MOS transistors occupying a reasonable silicon area. 

1. An integrated electronic circuit comprising a plurality of MOS transistors, with at least one first (19) and one second (20) MOS transistor arranged in series, each transistor comprising a grid and a short-circuit source, and a base connected to the ground of the integrated circuit, characterized in that it comprises a first resistance (15) mounted in parallel with said first transistor and a second resistance (16) mounted in parallel with said second transistor.
 2. The circuit as recited in claim 1, characterized in that it comprises a third transistor (21) mounted in series with said first and second transistors and comprising a grid and a short-circuit source, and a base junction connected to the ground of the integrated circuit.
 3. The circuit as recited in claim 2, characterized in that it comprises a third resistance (17) mounted in parallel with said third transistor.
 4. The circuit as recited in any one of the preceding claims, characterized in that it comprises a connection bump (18) that is connected to the source of said first transistor and is accessible after the circuit is encapsulated.
 5. The circuit as recited in any one of the preceding claims, characterized in that it comprises a connection bump (34) that is connected to the source of said second transistor and is not accessible after the circuit is encapsulated and a connection bump (35) that is connected to the drain of said second transistor and is not accessible after the circuit is encapsulated.
 6. The circuit as recited in any one of the preceding claims, characterized in that each MOS transistor occupies an area of less than 500 μm², preferably less than 200 μm².
 7. A method of adjusting electrical resistances in an integrated electronic circuit comprising a plurality of series-mounted MOS transistors, each provided with a parallel-mounted resistance, wherein the bases of the MOS transistors are connected to one another and a first voltage is applied to a MOS transistor at its base, its grid and its source and a second voltage is applied to its drain in order to break down said MOS transistor.
 8. The method as recited in claim 7, wherein said first voltage is constant and said second voltage is a monotonic ramp.
 9. The method as recited in claim 8, wherein said first voltage is zero and said second voltage is increasing. 